1. Field of the Invention
The present invention relates to a silver halide emulsion and a silver halide photographic light-sensitive material having a high sensitivity, an improved graininess, a high color density and hard photographic properties, resulting in excellent silver-saving properties.
2. Description of the Related Art
Emulsions having various outer shapes are known as silver halide emulsions constituting silver halide photographic light-sensitive materials. Examples are regular crystal emulsions containing, e.g., cubic, octahedral, tetradecahedral, and rhomboid dodecahedral grains, and twinned crystal emulsions containing double twinned crystals, such as tabular grains.
Among these emulsions, tabular grains constituting the twinned crystal emulsions have characteristics that light scattering is small owing to their outer shapes, a large amount of sensitizing dyes can be used because their specific surface areas are large, resulting in a high spectral sensitization efficiency. The characteristic features of the regular crystal emulsions, on the other hand, which are derived from their isotropic structures, are that formation of grains with, e.g., a multiple structure can be performed easily in accordance with the intended use, the emulsions can be monodispersed relatively easily, and spectral sensitization and chemical sensitization can be performed uniformly between grains. Therefore, the regular crystal emulsions are suitable for the purpose of providing hard-contrast emulsions with high color densities by increasing quantum sensitivities of the emulsions.
Representative examples of the regular crystal emulsions are a cubic emulsion whose surface is constituted by (100) faces and an octahedral emulsion whose surface is constituted by (111) faces. A variety of basic researches have long been made on these two types of emulsions. For example, as Tani describes in Photogr. Sci. Eng. 18:215-225 (1974), it is known that the intrinsic desensitization of the cubic emulsion with the (100) faces is smaller than that of the octahedral emulsion when sensitizing dyes are adsorbed. It is, therefore, considered that the cubic emulsion is superior to the octahedral emulsion as a spectral sensitizing emulsion.
It is known that the cubic emulsion can be easily formed with a silver halide primarily consisting of silver chloride. The manufacture of the cubic emulsion, however, is not necessarily easy with silver bromochloroiodide having a silver chloride content of 3 mol % or less, which is mainly used in high-sensitivity color photographic light-sensitive materials; the manufacture requires grain formation at a low pAg, that is difficult to control. If a silver halide solvent, such as ammonia, is used, the cubic emulsion can be formed even at a relatively high pAg. However, the presence of the solvent causes dissolution of the corners or the edges of grains to make it difficult to form a perfect cubic emulsion. On the other hand, when grain formation is performed at a low pAg or in the presence of ammonia, a reducing silver nucleus is formed in each silver halide grain. This sometimes results in undesirable photographic properties, such as production of fog. U.S. Pat. No. 3,655,394 discloses a method of manufacturing a cubic emulsion at a low pH and a relatively high pAg, under which conditions reducing silver nuclei are hard to form. In addition, JP-B-53-17492 ("JP-B" means Published Examined Japanese Patent Application), JP-B-57-56055, JP-B-60-35055, JP-A-62-115155 ("JP-A" means Published Unexamined Japanese Patent Application), JP-A-62-13250, and JP-A-2-87136 describe that a cubic emulsion can be manufactured at a high pAg or a sensitizing effect can be obtained when a specific compound is used together with the cubic emulsion. Although, however, a large number of examinations have been made on formation of cubic grains as described above, none of them completely solves the above problems.
In contrast, JP-A-62-229132 describes a cubic or tetradecahedral grain whose corners are rounded. When the present inventor performed supplementary tests, however, it was found that the sensitizing effect was obtained not by the rounded corners but by compounds which were added in order to round the corners.
Various studies have been made on the cubic emulsions as described above, but only few examples demonstrate the use of the emulsions as color photographic light-sensitive materials: the examples are some color negative films available from Eastman Kodak Co., Ltd., and motion-picture internegative films available from Eastman Kodak Co., Ltd. and Fuji Photo Film Co., Ltd.
According to the supplementary tests conducted on the patents described above by the present inventor, it was found that although nearly perfectly cubic grains could be made immediately after grain formation in some cases, those obtained through desalting and chemical sensitization, that were necessary in increasing the sensitivity, were all cubic grains whose corners were chipped. The present inventor has made further investigation but found no superiority of this imperfect cubic grain with chipped corners or edges to an octahedral emulsion and a tabular grain.
Moreover, silver halide color photographic light-sensitive materials have been recently required to have higher sensitivities and higher image qualities. In addition, for the purposes of saving resources, reducing cost, and decreasing quantities of replenishers of processing solutions, a strong demand has arisen for development of a silver halide color photographic light-sensitive material which can achieve a high color density even with a small silver amount without impairing image qualities, such as graininess.